A long pulse laser is defined herein as a laser with a pulse duration that exceeds the ultrasonic propagation time between the generation and detection locations at the surface of the workpiece. In the instance where the technique is applied to the detection of a discontinuity or flaw within or at the surface of the workpiece and where the generation and detection locations are on the same side of the workpiece and eventually superimposed, this propagation time is the time for ultrasound to propagate from the generation location to the discontinuity or flaw and subsequently from the discontinuity or flaw to the detection location; or, stated differently, it can be said that the pulse duration of a long pulse laser is sufficient in length to allow the capture of several ultrasonic echoes from such a discontinuity or flaw.
Ultrasonics is very powerful technique for inspecting and characterizing materials or industrial products. It allows the measurement of the thickness of parts to be determined, knowing the propagation velocity in the material. It is also widely used for detecting flaws in material. One important application is the detection of delaminations in polymer-matrix composite materials used in advanced aeronautic and aerospace structures. By performing several measurements along appropriate directions, ultrasonics also allows the determination of the elastic constants of the material. Furthermore, ultrasonics could be used for the determination of anisotropy or preferred crystallographic orientation (texture). One example of application is the determination of the texture of steel sheets by the generation of surface or plate waves in various directions. Texture is important for the deep drawing of such steel sheets, a forming process used for making parts of a car body or beverage cans.
Ultrasonics is usually applied by using piezoelectric transducers for the generation and detection of ultrasound. Ultrasonic coupling requires the transducers to be either in contact with the workpiece or to operate in immersion (usually in water). Such approaches are obviously not applicable at high temperatures. Another practical requirement is the orientation of the transducer in a direction approximately normal to the surface of the workpiece, which makes the inspection of pieces with complex geometry difficult or impossible. These limitations are eliminated by using lasers for the generation and detection of ultrasound. In this technique, called laser-ultrasonics (see prior art FIG. 1), a short pulse laser 10 is used for the generation of ultrasound. Pulse duration determines the frequency range of ultrasound that is generated, typically in the MHz range, thus durations of about 100 ns or shorter have been required. A long pulse laser 12 or a continuous laser is used for detection. A pulsed detection laser is preferable to a continuous laser, since it provides higher output power, it provides greater detection sensitivity. The duration of the pulse of the detection laser should be sufficiently long to capture all the ultrasonic echoes, which means, for most industrial applications, a pulse duration of more than 10 .mu.s is preferred. The requirement of generating frequencies in the MHz range apparently forbids the use of a long pulse laser (10 .mu.s and higher) for generation, and, as a consequence, prevents the use of the same laser for generation and detection. However, using the same laser for generation and detection would be desirable, reducing the laser hardware and the relatively high cost associated with this technique. As described later, the applicants have found that generation and detection of ultrasound with two long pulse lasers or even a single one is indeed possible and have shown that systems using such lasers could be applied to many situations of practical interest.
As will be described below, this invention is first based on the intensity modulation of the generation laser beam. Although modulated laser beams have been previously reported for the generation of ultrasound (see for example "Modulated laser array sources for the generation of narrowband and directed ultrasound", by J. W. Wagner, A. D. W. McKie, J. B. Spicer and J. B. Deaton, published in Journal of Nondestructive Evaluation, vol. 9, no 4, 1990, pp.263-270 and "Progress in pulsed laser array techniques for generation of acoustic waves", by T. W. Murray and J. W. Wagner, published in Review of Quantitative Nondestructive Evaluation, vol. 13, 1994, pp. 533-539), the duration of laser excitation has been typically relatively short, which is affirmed by the fact that in all the reported works the various ultrasonic echoes are resolved. The approach used in these reported works consists more specifically of using multiple short laser pulses in sequence and is the optical counterpart of the tone burst approach used in conventional ultrasonics. The practical interest of this approach is the increase of sensitivity which occurs at the repetition pulse frequency. The invention described in the present application applies instead to much longer pulse durations, which make the ultrasonic echoes to get mixed or the received signal to last a long time, comparable to the duration of the detection laser pulse. As a result, this requires an innovative approach which is described hereafter. It is also noted also that the use of a modulated long laser pulse (of duration comparable to the one considered here) for generation of ultrasound has been reported by R. Pierce, C. Ume and J. Jarzynski in Ultrasonics, vol. 33, no 2, 1995, pp. 133-137 (Title: Temporal modulation of a laser source for the generation of ultrasonic waves). This reported work uses conventional piezoelectric detection and not optical detection and does not address the problem of overlapping echoes, since a particular detection configuration and a special test block was used.
Modulated laser beams have also been used for photodisplacement imaging and there have been reports of the use of a single laser for generation and detection (see "Photodisplacement techniques for defect detection"by Y. Martin and E. A. Ash published in Philosophical Transactions of the Royal Society of London A (Mathematical and Physical Sciences), vol. 320, 1986, pp. 257-269 and "New technique of photodisplacement imaging using one laser for both excitation and detection"by L. Chen, K. H. Yang and S. Y. Zhang published in Applied Physics Letters, vol. 50, 1987, pp. 1349-1351). It should first be noted that photodisplacement imaging is based on the detection of the initial surface deformation produced by laser absorption and not on the detection of ultrasound. This technique is a photothermal technique and subsurface defects are revealed by their interaction with the generated thermal wave. In these reported works, essentially single frequency modulation is used and surface displacement is measured at the output of a detection interferometer by monitoring a term at the second harmonic of the modulation frequency. The invention described in the present application applies instead to the detection of ultrasound and is not limited to essentially single frequency modulation. Measurement of surface deformation by monitoring a second harmonic term as described in these previous works restricts the usable modulation bandwidth to range from a given frequency to its second harmonic and is consequently of limited use. The invention described here does not rely on this approach.
Other important background information relevant to this invention relates to the method used for extracting the ultrasonic information of interest from the beam originating from the detection laser and reflected or scattered by the surface of the workpiece. Ultrasonic excitation of the object produces at its surface, small displacements which cause a phase or frequency perturbation on the scattered or reflected beam. Since the displacements are quite small, the optical phase or frequency discriminator has to be very sensitive; thus, in practice should be based on optical interferometry. Furthermore, the probed surfaces are rough, so the ultrasonic information is encoded into an optical beam with speckle and a suitable interferometric technique should integrate effectively over the whole speckle field or provide demodulation independently of speckle nature of the collected light beam. In various US patents the applicant (J.-P. Monchalin) has described interferometric schemes for sensitive detection in these conditions. In an arrangement described by the applicant in U.S. Pat. No. 4,659,224 issued Apr. 21, 1987, entitled "Optical Interferometric Reception of Ultrasonic Energy", a confocal Fabry-Perot is used in transmission to provide a signal representative of the surface motion independently of the speckle effect. In U.S. Pat. No. 4,966,459 issued Oct. 30, 1990, entitled "Broadband Optical Detection of Transient Surface Motion From a Scattering Surface", J.-P. Monchalin describes the use of the same type of interferometer, that may be used within a Mach-Zehnder interferometric arrangement or in a reflection scheme to provide the same capability with a very broad detection bandwidth. Still, a broader detection bandwidth, especially including the low ultrasonic frequency range, several kHz to about 1 MHz, is described by J. P. Monchalin and R. K. Ing in U.S. Pat. No. 5,131,748 issued Jul. 21, 1992, entitled "Broadband Optical Detection of Transient Motion from a Scattering Surface"and based on the use of two-wave mixing in a photorefractive crystal. Still another approach using a photorefractive crystal is described by I. A. Sokolov, S. I. Stepanov and G. S. Trofimov, in the Journal Opt. Soc. Am. B, Vol. 9, No. 1, January 1992, p.p. 173-176. In this approach the crystal performs the optical phase demodulation without the use of an optical detector and a voltage (photo-electromotive force) representative of the phase modulation of the detected signal beam appears on two electrodes at the surface of the crystal.
It is an object of the invention to provide a preferred laser ultrasonic apparatus and method for inspecting and characterizing materials.